WO 97/23762 discloses a method for layer thickness measurement which is based on an electrically conductive layer being present on a base material; the conductivities of layer and base material must be different. The ratio of the conductivity of layer and base material is limited to a ratio of from 0.7 to 1.5. Moreover, in particular eddy currents in a range from 1.5 to 3.5 MHz are used.
GB 22 79 75 A1 describes conductivity measurements in the region close to the surface for materials with a high magnetic permeability, which can only be tested in magnetic saturation.
U.S. Pat. No. 5,793,206 describes a measurement probe for layer thickness measurement.
These methods and/or the probes have the drawback of only being able to determine layer thicknesses.
The book “Werkstoffprüfung” [materials testing], 5th Edition, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig 1989, by H. Blumenauer, describes the nondestructive testing of materials using the eddy current method. It is based on the electromagnetic alternating field of a coil through which alternating current is flowing changing when a metallic specimen is brought into its region of action. The primary field of the coil induces an AC voltage in the specimen that is to be tested, and this voltage in turn generates an alternating current, which for its part then generates a magnetic alternating field. This secondary alternating field counteracts the primary field in a characteristic way and thereby changes its parameters. The change can be recorded by measurement means. For this purpose, for example in the case of coils with a primary and secondary winding, the secondary voltage is measured (transformer principle). Alternatively, for example in the case of coils with only one winding, the impedance thereof is determined (parametric principle). According to the laws which apply in an alternating current circuit, the induction in the coil and in the specimen, in the case of the parametric arrangement, in addition to the ohmic resistance, also generates an inductive resistance and, in the case of the transformer arrangement, generates an imaginary measurement voltage as well as the real measurement voltage. Both components are presented in complex form in the impedance plane or the complex voltage plane. In both these examples, the nondestructive testing of materials makes use of the effect whereby the changes in the primary field are dependent on the physical and geometric specimen properties and also on the apparatus properties. Apparatus properties include the frequency, the current intensity, the voltage and the number of windings of the coil. Specimen properties include electrical conductivity, permeability, specimen shape and material inhomogeneities in the region of the eddy currents. More recent appliances for inductive testing allow measurements at a plurality of excitation frequencies. For this purpose, by way of example, the frequency during a measurement can be altered automatically, or the frequency is adjusted manually by the user during two measurements. The frequency has a considerable influence on the penetration depth of the eddy currents. Approximately the following relationship applies:
  δ  =      503                  f        ·        σ        ·                  μ          r                    
[mm] penetration depth,
f[Hz] frequency,
[MS/m=m/(Ωmm2)] specific conductivity,
μr relative permeability.
The standard penetration depth decreases as the frequency rises.
The article “Non-Destructive Testing of Corrosion Effect on High Temperature Protective Coatings” by G. Dibelius, H. J. Krischel and U. Reimann, VGB Kraftwerkstechnik 70 (1990), No. 9, describes the nondestructive testing of corrosion processes in protective layers on gas turbine blades and vanes. A measurement method used for nickel-based protective layers is measurement of the magnetic permeability, on account of the ferromagnetism changing during the corrosion process, i.e. the material has a very high relative magnetic permeability (>100-1000), in the protective layer. The possibility of eddy current measurement is discussed for the case of a platinum-aluminum protective layer system. The layer thickness of the protective layer can be worked out on the basis of the measured signal levels.
The article “How to cast Cobalt-Based Superalloys” by M. J. Woulds in: Precision Metal, April 1969, p. 46, and the article by M. J. Woulds and T. R, Cass, “Recent Developments in MAR-M Alloy 509”, Cobalt, No. 42, pages 3 to 13, describe how the solidifying or solidified component surface can react with the material of the cast shell during the casting of components and also gas turbine blades and vanes. This can, for example, lead to the oxidation of carbides in the cast component. A phenomenon of this type is also referred to here as “Inner Carbide Oxidation”, ICO. The formation of ICO leads to carbides which reinforce the grain boundaries of an alloy breaking down.
In particular in the region of a gas turbine blade or vane which is close to the surface, this can lead to considerable weakening of the material. The alloys are usually cast using vacuum casting. The oxygen which is required for oxidation is derived from the material of the casting shell, e.g. silicon dioxide, zirconium dioxide or aluminum oxide. As a result, oxide phases are formed on the grain boundaries. The original carbides are transformed, for example, into zirconium-rich, titanium-rich or tantalum-rich oxides. The depth of the region which contains oxidized carbides is dependent on parameters such as the carbon content in the alloy, the composition of casting shell material and casting alloy, and also the cooling rate. An oxide-containing layer of this type may typically be approximately 100 to 300 μm thick. For quality control, it is desirable for it to be possible to detect the oxide regions of oxidized carbides which have an adverse effect on the mechanical properties. This has not hitherto been possible by nondestructive testing.
Under certain environmental conditions, nickel- and cobalt-base alloys tend to form a form of corrosion known as high-temperature corrosion (HTC). From a materials science perspective, HTC is a complex form of sulfiding of the base material which takes place at the grain boundaries. As HTC progresses, supporting cross sections of components are weakened. It is important to know the depth of the HTC attack in order to allow the operating safety and remaining service life of a component to be estimated and in order to decide whether rework (e.g. refurbishment of gas turbine blades or vanes) is possible.